Gas Heating System; General; Fuel Types And Pressures - Carrier 48FC M20 Series Service And Maintenance Instructions

GAS HEATING SYSTEM

General

The heat exchanger system consists of a gas valve feeding multi-
ple in-shot burners off a manifold. The burners fire into matching
primary tubes. The primary tubes discharge into combustion ple-
num where gas flow converges into secondary tubes. The second-
ary tubes exit into the induced draft fan wheel inlet. The induced
fan wheel discharges into a flue passage and flue gases exit out a
flue hood on the side of the unit. The induced draft fan motor in-
cludes a flue gas pressure switch circuit that confirms adequate
wheel speed through the Integrated Gas Control (IGC) board.
Safety switches include a Rollout Switch (located in the burner
section; see Fig. 52) and a limit switch (mounted through the side
of the heat exchanger cover panel [Gas Limit Switch Side Shot]
and through the vestibule plate [Gas Limit Switch Down Shot]).
(See Fig. 46.)
Gas Limit
Switch
Side Shot
Gas Limit Switch
Down Shot
Fig. 46 — Gas Limit Switch Location

Fuel Types and Pressures

NATURAL GAS
The 48FC unit is factory-equipped for use with natural gas (NG)
fuel at elevation under 2000 ft (610 m). See section "Orifice Re-
placement" for information in modifying this unit for installation
at elevations above 2000 ft (610 m).
Gas line pressure entering the unit's main gas valve must be within
specified ranges (see Table 5). Adjust unit gas regulator valve as
required or consult local gas utility.
Table 5 — Natural Gas Supply Line Pressure Ranges
UNIT MODEL UNIT SIZE
48FC** 20, 24, 28, 30
Manifold pressure is factory-adjusted for NG fuel use. Adjust as
required to obtain best flame characteristic (see Table 6).
Table 6 — Natural Gas Manifold Pressure Ranges
UNIT MODEL UNIT SIZE
48FC** 20, 24, 28, 30
LIQUID PROPANE
Accessory packages are available for field-installation that will
convert the 48FC unit to operate with liquid propane (LP) fuels.
These kits include new orifice spuds, new springs for gas valves
and a supply line low pressure switch. See High Altitude Gas
Conversion Kit Gas Heating/Electric Cooling 3-15 Ton Small
Rooftop Units Accessory LP (Liquid Propane) Installation In-
structions for details on orifice size selections.
Fuel line pressure entering unit gas valve must remain within
specified range (see Table 7).
MIN. MAX.
5.0 in. wg 13.0 in. wg
(1246 Pa) (3240 Pa)
HIGH FIRE LOW FIRE
3.0 in. wg 2.0 in. wg
(748 Pa) (498 Pa)
Table 7 — Liquid Propane Supply Line Pressure
UNIT MODEL UNIT SIZE
48FC** 20, 24, 28, 30
Manifold pressure for LP fuel use must be adjusted to specified
range (see Table 8). Follow instructions in the accessory kit to
make initial readjustment.
Table 8 — Liquid Propane Manifold Pressure Ranges
UNIT MODEL UNIT SIZE
48FC** 20, 24, 28, 30
SUPPLY PRESSURE SWITCH
The LP conversion kit includes a supply low pressure switch.
The switch contacts (from terminal C to terminal NO) will open
the gas valve power whenever the supply line pressure drops be-
low the set point. See Fig. 47 and 48. If the low pressure remains
open for 15 minutes during a call for heat, the IGC circuit will
initiate a Ignition Fault (5 flashes) lockout. Reset of the low pres-
sure switch is automatic on rise in supply line pressure. Reset of
the IGC requires a recycle of unit power after the low pressure
switch has closed.
Fig. 47 — LP Low Pressure Switch (Installed)
Fig. 48 — LP Supply Line Low Pressure
This switch also prevents operation when the propane tank level
is low, which can result in gas with a high concentration of im-
purities, additives, and residues that have settled to the bottom of
the tank. Operation under these conditions can cause harm to the
heat exchanger system. Contact your fuel supplier if this condi-
tion is suspected.
37
Ranges
MIN.
11.0 in. wg 13.0 in. wg
(2740 Pa)
HIGH FIRE LOW FIRE
11.0 in. wg
(2740 Pa)
Switch Wiring
MAX.
(3240 Pa)
7.3 in. wg
(1819 Pa)